In https://www.gnu.org/software/grub/manual/grub/grub.html (I am showing
the page source, lines 3058 through 3068):
<dt>‘<samp>left</samp>’</dt>
dd><p>The distance from the left border of container to left border of
the object in either of three formats:
</p><table>
<tr><td width="20%">x</td><td width="70%">Value in pixels</td></tr>
<tr><td width="20%">p%</td><td width="70%">Percentage</td></tr>
<tr><td width="20%">p%+x</td><td width="70%">mixture of both</td></tr>
</table>
</dd>
<dt>‘<samp>top</samp>’</dt>
<dd><p>The distance from the left border of container to left border of
the object in same format.
</p></dd>
The definition of 'left' and 'top' are the same; the 'top' one is
obviously incorrect. What is tricky is that I think 'top' is the
distance from the bottom border of the container to the top border of
the object.